The field of this disclosure generally relates to cathode material for use in thermal batteries and, particularly, to cathode material that includes a primary active material and an amount of a bi-metal sulfide such as, for example, CuFeS2. The disclosure also relates to batteries (e.g., thermal batteries) that contain such cathode materials.
Thermal batteries tend to have relatively long shelf lives, high energy densities, require relatively low maintenance, and can withstand relatively high temperatures. Thermal batteries also tend to provide a short burst of power over a relatively short period of time. The burst may range from less than a second to an hour or more, with power typically ranging from about a watt or less to kilowatts. Such properties make thermal batteries suitable for military (e.g., batteries for missile guidance systems) and space exploration applications. Thermal batteries may also be used in other applications, such as in electric vehicles.
A typical thermal battery includes an anode, a cathode, an electrolyte-separator containing a solid electrolyte that is non-conductive at ambient temperature, and a pyrotechnic material (e.g., heat pellet as in FIG. 1 which may contain, for example, Fe—KClO4 powder) that provides a heat source to the battery. When battery operation is desired, an external stimulus is applied to the battery. For example, an electrical current may be applied to the battery to set off an electric match or an electro-active squib or a mechanical force (e.g., mechanical shock) may be applied to set off a concussion primer. The external stimulus causes the pyrotechnic material to ignite and begin to heat. Heat produced from the pyrotechnic material causes the previously solid electrolyte to melt and become conductive, which allows the battery to provide power for a desired application.
Thermal batteries are often formed using pellet techniques, such that each of the electrolyte, cathode, and heat source are formed into a wafer. In this case, the respective cell component chemicals are processed into powders and the powders are pressed together to form the cell. Each component may be formed as a discrete part, or the anode and/or cathode may include (i.e., be flooded with) electrolyte material to improve the conductivity of the cell.
The anodes of thermal batteries are generally formed of an alkali or alkaline earth metal or alloy. A typical anode includes lithium metal or a lithium alloy, such as lithium aluminum, lithium silicon, or lithium boron.
Electrolytes for use with thermal batteries often include a eutectic mixture (i.e., a mixture which solidifies at a temperature lower than each of the individual components) of lithium chloride and potassium chloride and a binder, such as MgO, fumed silica or clay minerals such as kaolinite (including kaolin clays which are known to be rich in kaolinite), which assists in containing the electrolyte within the thermal battery assembly such as by capillary action, surface tension, or both. The electrolyte-separator is often composed of binary or ternary salts melting at temperatures above ambient between 200° C. and 600° C. With typical thermal battery electrolytes, without sufficient binder, the electrolyte material may disperse throughout the battery, causing undesired shunts or short circuits in the cell.
Cathode material for thermal batteries may vary in accordance with a variety of design parameters and generally includes a metal oxide or metal sulfide. By way of example, iron oxide, iron disulfide or cobalt disulfide are often used as cathode material.
Typical thermal batteries make use of what is essentially a monolithic cathode material. While the cathode may contain components other than active cathode material such as, for example, the electrolyte to provide flooding and a lithiation additive (i.e., a lithium compound other than a lithium salt) to control voltage, conventionally there is only one active material such as, for example, a metal oxide (e.g., FeO4) or metal sulfide (e.g., CoS2 or FeS2). Some research has been performed on incorporating additives of other sulfides to provide improved performance, such as Walsh et al. in U.S. Pat. No. 3,992,222, who examined FeS2 cathodes incorporating a second sulfide as an additive to result in improved performance. The sulfides that were examined, however, were limited to single metal sulfides, such as titanium disulfide, nickel sulfide, or cerium sulfide.
A continuing need therefore exists for cathode materials that contain additives that result in improvements in conductivity, voltage and lifetime. A continuing need also exists for thermal batteries that incorporate such cathode materials and that exhibit such improved performance.